How to prepend/append text to any field in `biblatex.cfg` file?

Question

I want a portable way to prepend/append text to any field of any item in the bibliography, which means that it has to go in biblatex.cfg which is a special file that is automatically read when using biblatex package.

biblatex.cfg (can't be generated with filecontents):

\ExecuteBibliographyOptions{sorting=nyt}
\DeclareSourcemap{
  \maps{
    \map[overwrite]{
      \step[fieldsource=url, final]
      % \step[fieldset=author, fieldvalue={|prepend|}, prepend]
      \step[fieldset=author, fieldvalue={|append|}, append] % That actually works
    }
  }
}

MWE (for lualatex):

\documentclass{article}
\begin{filecontents}{example.bib}
@book{sometag,
  author = {author},
  title = {title},
  year = {year},
  url = {url},
}
\end{filecontents}
\usepackage{biblatex}
\addbibresource{example.bib}
% I don't understand why it doesn't work but it's not portable anyway.
% \usepackage{xpatch}
% \xpretofieldformat{author}{|prepend|}{}{}
% \xapptofieldformat{author}{|append|}{}{}
\nocite{*}
\begin{document}
\printbibliography{}
\end{document}

Output:

Desired output:

Answer 1

I don't think there is a general way to prepend and append to arbitrary fields in biblatex.

A sourcemap not dissimilar to the one shown in the question is the first thing I would think of as the most universal solution. While append exists (and will work as shown in your example*), there is no prepend, but such a feature could be emulated with regexp matching.

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{biblatex}
\DeclareSourcemap{
  \maps{
    \map[overwrite]{
      \step[fieldsource=title,
            match=\regexp{\A(.*)\Z},
            replace=|pre text|$1|post text|]
    }
  }
}
\begin{filecontents}{\jobname.bib}
@book{sometag,
  author = {author},
  title  = {title},
  year   = {2023},
  url    = {url},
}
\end{filecontents}
\addbibresource{\jobname.bib}
\nocite{*}
\begin{document}
\printbibliography
\end{document}

For simple text like pre text and post text you can just use

replace=|pre text|$1|post text|

if you have more complex macros in there that are not expandable (or want to use some RegEx features), you are better off wrapping the text in \regexp, but then spaces are eaten by the normalisation, so you have to use \x{20} instead and the regex meaning of characters is active, which means you need to escape \ and the like

replace=\regexp{|pre\x{20}\\textbf{text}|$1|post\x{20}text|}

will produce

in the above example.

A sourcemap solution comes with drawbacks, however.

• The text is added to the field when the .bib file is read. That means it influences sorting and all other .bib-data-based operations.

• The text becomes part of the field, which means it will be formatted like the field. In the example the "pre" and "post" are also italicised.

• For name list fields like author, the added text is parsed as part of the name. This may lead to extremely odd results as in

  \documentclass{article}
  \usepackage[T1]{fontenc}
  \usepackage{biblatex}
  \DeclareSourcemap{
    \maps{
      \map[overwrite]{
        \step[fieldsource=author,
              match=\regexp{\A(.*)\Z},
              replace=|pre|$1|post|]
      }
    }
  }
  \begin{filecontents}{\jobname.bib}
  @book{sometag,
    author = {Jane Doe and Elk, Anne},
    title  = {title},
    year   = {2023},
    url    = {url},
  }
  \end{filecontents}
  \addbibresource{\jobname.bib}
  \nocite{*}
  \begin{document}
  \printbibliography
  \end{document}
  

  

  This is not something you can counter easily because you don't know how the names are formatted.

So for most intents and purposes the sourcemap is probably not going to cut it.

All other solutions, however, will be style-dependent to some degree.

• All biblatex standard styles and almost all well-written third-party styles have a begentry and finentry bibmacro that is executed at the beginning and end of the entry, respectively. They can be redefined to print arbitrary text at that point. An example of begentry can be found in How to print shortauthor in the references as well?.

• There are usually two ways a style can take to print text before or after a field. Depending on whether or not that text should be considered "part of the field output" one of these solutions might be more appropriate than the other.

  • Find out which bibmacro prints the field and insert the text before or after the relevant \printfield, \printlist or \printnames call. Obviously that is highly style-dependent.

  • Use field formats to add the text, e.g. \DeclareFieldFormat{title}{pre-#1-post}. This will overwrite what the style does and may only half-work as expected due to type-specific formatting (cf. Remove Quotation Marks from Style).

Everything discussed here can be used in biblatex.cfg, but obviously not all of the solutions work the same for all possible styles you would want to use.

---

In the end I think we can only give you a useful answer if you can describe your actual use case (or at least the motivation for asking this question) in more detail. The point of biblatex is to display your .bib data in format defined in the style you use. It's not to insert arbitrary text in there - unless that's what the style wants (but then you need to customise the specific style you want to use).

---

* The example does not work on my machine, because filecontents refuses to write out biblatex.cfg, since a global biblatex.cfg already exists. Apparently LaTeX thinks it would overwrite that file. If I generate the file with the shown contents manually, I get an error about prepend, but when I delete that line, the append works as expected.

Answer 2

I want to share my solution in case someone will have a similar problem.

In reality, I only need to prepend to the beginning of the item some text if the url field is present. But the selected answer apparently didn't fit my case. This is because some of my sources don't have an author field, and therefore the solution becomes much harder/bigger. Well, that is, if the solution isn't this one:

% Solution 1
\def\prepend{prepend} % Any text goes here
\renewbibmacro{begentry}{\iffieldundef{url}{}{\prepend\addspace}}

But, for example, this one:

I've found the definition of \newbibmacro*{author} and \newbibmacro*{title} in texlive/20*/texmf-dist/tex/latex/biblatex/biblatex.def (which appeared in logs):

\newbibmacro*{author}{%
  \ifboolexpr{
    test \ifuseauthor
    and
    not test {\ifnameundef{author}}
  }
    {\printnames{author}%
     \iffieldundef{authortype}
       {}
       {\setunit{\printdelim{authortypedelim}}%
        \usebibmacro{authorstrg}}}
    {}}

\newbibmacro*{title}{%
  \ifboolexpr{
    test {\iffieldundef{title}}
    and
    test {\iffieldundef{subtitle}}
  }
    {}
    {\printtext[title]{%
       \printfield[titlecase]{title}%
       \setunit{\subtitlepunct}%
       \printfield[titlecase]{subtitle}}%
     \newunit}%
  \printfield{titleaddon}}

Then I redefined (and reformatted) them like this:

% Solution 2
\def\prepend{prepend} % Any text goes here
\renewbibmacro*{author}{%
  \ifboolexpr{
    test \ifuseauthor
    and
    not test {\ifnameundef{author}}
  }{%
    \iffieldundef{url}{}{\prepend\addspace}%
    \printnames{author}%
    \iffieldundef{authortype}{}{%
      \setunit{\printdelim{authortypedelim}}%
      \usebibmacro{authorstrg}%
    }%
  }{}%
}
\renewbibmacro*{title}{%
  \ifboolexpr{
    test {\iffieldundef{title}}
    and
    test {\iffieldundef{subtitle}}
  }
  {}
  {%
    \ifboolexpr{% Simple \iffieldundef{author}{}{} didn't work in some cases
      test \ifuseauthor
      and
      not test {\ifnameundef{author}}
    }{}{%
      \iffieldundef{url}{}{\prepend\addspace}%
    }%
    \printtext[title]{%
      \printfield[titlecase]{title}%
      \setunit{\subtitlepunct}%
      \printfield[titlecase]{subtitle}%
    }%
    \newunit%
  }%
  \printfield{titleaddon}%
}

Now, even if author does not exist (but title does) then the \prepend text is added to the left.

@book{sometag1,
  author = {author1},
  title = {title1},
  year = {year1},
}
@book{sometag2,
  author = {author2},
  title = {title2},
  year = {year2},
  url = {url2},
}
@book{sometag3,
  author = {author3},
  title = {title3},
  year = {year3},
}
@book{sometag4,
  title = {title4},
  year = {year4},
}
@book{sometag5,
  title = {title5},
  year = {year5},
  url = {url5},
}